1. Field of the Invention
The present invention relates to an ink tank for an ink jet recording device, which inhibits penetration of a gas and which is suitable for a small-sized flexible ink jet recording device having a high use efficiency of ink.
2. Description of the Related Art
An ink jet recording device includes an ink jet recording head which flies ink as liquid droplets in response to an applied electric signal, and the ink is supplied to the head from an ink reservoir referred to as an ink tank or an ink cartridge through an ink channel. During the supply of the ink, it is demanded that an appropriate negative pressure be maintained with respect to the recording head, evaporation of the ink be prevented to inhibit a change of a physical property and generation and mixture of bubbles in the ink be inhibited to smoothly distribute the ink.
The inhibition of the evaporation of the ink is noted in a supply system of the ink. For example, when several percentages of a volatile component of the ink evaporate, a substantially usable amount of the ink decreases. In addition, viscosity increases, and the ink is not smoothly discharged from the ink jet recording head. When a component that easily volatilizes preferentially evaporates and a ratio between a polar solvent and a non-polar solvent fluctuates, the ink is sometimes separated. When an image is output with the ink having its properties changed in this manner, a long time is required for drying the ink attached to paper, and the ink migrates. As a result, problems such as color mixture and deterioration of a resolution of the image occur in some case.
In general, a plastic is used as a material of an ink storage section of the ink tank (Japanese Patent Application Laid-Open No. H07-323570). Since an olefin-based resin such as polypropylene is chemically stable, inexpensive and easily moldable by injection molding, the resin is frequently used. High-impact polystyrene, noryl resin and polysulfone have a gas penetration ratio higher than that of the olefin-based resin, and have a slightly deteriorated resistance to evaporation, but are used in a case where a mechanical strength higher than that of the olefin-based resin is required. Furthermore, even when a bag made of such a plastic is used in the storage section of the ink and the stored ink of a pigment sedimentation system needs to be periodically stirred, an external force can easily be applied to stir the ink.
However, in the ink storage section of the bag made of such a plastic, the gas penetrates the resin. When the ink is volatilized or consumed, outside air invades an inner space of the storage section to cause a problem. As shown in, for example, FIGS. 7A, 7B and 7C, in an ink tank having an outer vessel 701 and an ink storage section 702 made of a plastic and disposed in the vessel, a bubble 704 of a gas volatilized from an ink component is generated in the ink storage section in which ink 703 is stored (FIG. 7A). Outside air 706 invades a portion 705 of the ink storage section which comes into contact with this bubble to enlarge the bubble 704 (FIG. 7B). Furthermore, a volume of the bubble continues to be enlarged to increase an inner pressure (FIG. 7C). In the worst case, the ink storage section 702 cannot bear the inner pressure, and is sometimes damaged.
It is known that a mechanism in which the outside air invades this plastic bag is caused by an osmic pressure generated by a difference between a vapor concentration of the ink component of the bubble 704 in the ink storage section and a vapor concentration of the ink component included in the outside air.ΔP=(n1−n2)RT: 
n1: ink vapor mol concentration [mol/l] in the ink storage section;
n2: ink vapor mol concentration [mol/l] in the outside air; and
R: gas constant (0.082 [(atm/K)·mol].
Specifically, ΔP indicates that the outside air penetrates a bag member to invade the ink storage section owing to a function of reducing a vapor pressure difference of the ink component between the inside and the outside of the ink storage section, that is, a function of reducing a molar fraction of the ink component in the section. However, the vapor pressure of the ink component in the ink storage section is sufficiently higher than that of the ink component in the outside air. Therefore, ΔP of the above equation is semi-permanently present, and inflow of the air semi-permanently continues.
To solve such a problem of the invasion of the outside air into the ink storage section of the resin bag, a method is reported in which a vapor density difference between the inside and the outside of the ink storage section is reduced in a stepwise manner by use of a wall having a double structure (Japanese Patent Application Laid-Open No. H06-135000).
Moreover, for the ink storage section of the bag made of the plastic resin, a substance having a small gas penetration ratio is selected. The section is constituted by laminating a film of a metal such as aluminum to reduce the gas penetration ratio. A so-called laminated film is used in the section.
However, to obtain a sufficient effect of preventing the inflow of the outside air, such a laminated film requires a plastic film having a thickness of about 50 μm at minimum and an aluminum foil having a thickness of about 10 μm. The film is formed of a remarkably hard material which is not easily bent. As a result, it is difficult to reduce a capacity of the ink storage section with the consumption of the ink. There is a problem that all the stored ink cannot be used. A so-called use-up property deteriorates.
Furthermore, in the ink tank, the ink bag is expanded using a force of a spring to apply the negative pressure to the ink. Since the ink bag formed of the laminated film has a high mechanical rigidity, it is difficult to adjust the capacity of the ink bag with the consumption of the ink. As a result, the negative pressure applied to the ink tends to be unstable. Furthermore, it is difficult to sort the laminated film during disposal of the film as garbage. Therefore, there is a problem of resource protection.